The present invention relates to a code division multiple access (CDMA) communication system, and more particularly to a method for enhancing voice quality of a CDMA communication system transmitting a voice signal and a signaling message by control of an encoding rate of a variable rate vocoder.
In a CDMA communication system, a speech signal or voice signal from a microphone is encoded by a vocoder. The vocoder has variable encoding rates corresponding to the speech signal, and outputs encoded voice data depending on the encoding rate. A central processing unit (CPU) provides a signaling message or a transmit message, which includes information such as commands and control signals required to communicate with a base station, by detecting an encoding rate of the voice data. Thus, the CPU controls the encoding rate of the vocoder to transmit the signaling message.
The signaling message is formatted to a specific format to be transmitted as a packet composed of the signaling message itself, or composed of the signaling message and the encoded voice data. The signaling message and the encoded voice data are transmitted through a traffic channel.
The variable rate vocoder included in the CDMA communication system has various encoding rates. Generally, the vocoder encodes the voice data at one of the encoding rates, such as a full rate, a half (xc2xd) rate, a quarter (xc2xc) rate, and an eighth (xe2x85x9) rate. For transmitting the voice data through the CDMA traffic channel, the vocoder selects a suitable encoding rate corresponding to the speech signal from a user to prevent degradation of voice quality. In addition, the vocoder determines whether the selected encoding rate is between a maximum encoding rate and a minimum encoding rate received from the CPU, and adjusts the selected encoding rate to the maximum/minimum encoding rate when the selected encoding rate is not between the maximum and minimum encoding rates. The CPU restricts the encoding rates of the vocoder to transmit the signaling message.
FIG. 1 is a block diagram illustrating a conventional CDMA mobile station 10 with a variable rate vocoder 14. Referring to FIG. 1, the CDMA communication system 10 comprises a coder-decoder (CODEC) 12, the variable rate vocoder 14, a CPU 16, a digital modulator 18, a baseband analog (BBA) processor 20, and a radio frequency (RF) transmitter 22.
The CODEC 12 converts an analog speech signal from a microphone to a digital speech signal. The variable rate vocoder 14 encodes the digital speech signal at one of various encoding rates corresponding to a characteristic of the speech signal and outputs encoded voice data to the CPU 16. The CPU 16, running under control of a control program, processes the encoded voice data from the vocoder 14 and transmits the processed voice data to the digital modulator 18. The digital modulator 18 modulates a packet of data received from the CPU 16 and spreads the spectrum of the modulated packet data. The BBA processor 20 coupled between the digital modulator 18 and the RF transmitter 22 receives the spread spectrum data from the digital modulator 18 and converts it to a baseband signal. The RF transmitter 22 receives the baseband signal and converts it to an RF signal. The RF transmitter 22 amplifies the RF signal and outputs the amplified RF signal to an antenna for communication to a base station.
In the above described process, if a frame packet including the encoded voice data is not sufficiently large to insert the signaling message, the CPU 16 controls the vocoder 14 to encode the voice data at a lower encoding rate compared to a present encoding rate. When the vocoder 14 is operated at the lower rate, a frame has a packet assembly for transmitting both the voice data and the signaling message.
For example, when the present frame contains the signaling message to be transmitted, if the digital speech signal is encoded to above 80 bits by the vocoder 14 at the full rate, the remaining bits, i.e., the bits other than the encoded 80 bits cannot be transmitted in the present frame. To solve this problem, the CPU 16 controls the vocoder 14 to operate at a lower encoding rate, for example, a xc2xd, xc2xc or xe2x85x9 rate, to encode the voice data within the 80 bits. As a result, the voice quality of the CDMA communication system is degraded.
FIGS. 2A and 2B are diagrams illustrating frame formats for the various data rates, types and modes of rate set 1. These frame formats are illustrated in U.S. Pat. No. 5,568,483 to Padovani et al., issued on Oct. 22, 1996, entitled, xe2x80x9cMETHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION.xe2x80x9d Referring to FIG. 2A, frames A-E are provided at a 9,600 bps (i.e., 9.6 kbps) rate, consist of 192 bits in length, and span 20 ms in time. Frames A-E include a single mixed mode bit MM, auxiliary format bits if present, message bits, a 12-bit frame quality indicator F, and 8 tail bits T. The frame formats shown FIGS. 2A and 2B are well known to a person of ordinary skill in the art, so that their detailed explanation will be omitted.
As shown in FIG. 2A, frame A is a 9,600 bps frame for the transmission of primary traffic only, which includes user speech. Frame B is a 9,600 bps dim and burst frame for the transmission of xc2xd rate primary traffic and signaling traffic. Frame C is a 9,600 bps dim and burst frame for the transmission of xc2xc rate primary traffic and signaling traffic. Frame D is a 9,600 bps dim and burst frame for the transmission of xe2x85x9 rate primary traffic and signaling traffic. Frame E is a 9,600 bps blank and burst frame for the transmission of signaling traffic, such as command and control signals.
Referring to FIG. 2B, frame F is a 4,800 bps (i.e., 4.8 kbps) frame for the transmission of xc2xd rate primary traffic only. Frame G is a 2,400 bps (i.e., 2.4 kbps) frame for the transmission of xc2xc rate primary traffic only. Frame H is a 1,200 bps (i.e., 1.2 kbps) frame for the transmission of xe2x85x9 rate primary traffic only.
The encoding rate of the next frame of the vocoder 14 is defined by the CPU 16 in response to the various encoding rates of the present frame or an existence of the signaling message. Therefore, the CPU 16 can output the voice data and/or signaling message to the digital modulator 18 in an appropriate frame packet format, which is selected from the frame packet formats shown in FIGS. 2A and 2B.
For example, if the present frame is a normal frame, the signaling message to be transmitted does not exist. Thus, the vocoder 14 generates the packet composed of only voice data, in response to one of the various encoding rates corresponding to characteristics of the voice data being encoded. Frame A of FIG. 2A and frames F, G and H of FIG. 2B correspond to this case. These frames are transmitted at 9,600 bps, 4,800 bps, 2,400 bps and 1,200 bps rates, respectively. In that case, the voice quality of the CDMA communication system is not affected.
If the present frame is a blank and burst frame, the voice data to be transmitted does not exist. In that case, the packet is composed of only a signaling message, so that the vocoder 14 generates the packet composed of only a signaling message. Frame E of FIG. 2A corresponds to this case. In that case, the voice quality is not affected.
And if the present frame is a dim and burst frame, the maximum number of bits capable of loading the voice data as the primary traffic bits are restricted to 80 bits as shown for example as frame B of FIG. 2A. In that case, if the voice data is encoded at the full rate (i.e., more than 80 bits) although the signaling message to be transmitted exists in the present frame, the remaining bits excepting the 80 bits cannot be transmitted in the present frame. To prevent this problem, the encoding rate of the variable rate vocoder 14 is restricted within the xc2xd rate under control of the CPU 16, so as to encode the voice data within 80 bits. Thus, in this case, the voice quality of the CDMA communication system is degraded. Frames B, C and D of FIG. 2A correspond to this case.
As described above, when the vocoder of the conventional CDMA communication system transmits both encoded voice data and the signaling message, the encoding rate of the vocoder is restricted under compulsion by the CPU. Thus, the voice quality of the CDMA communication system is degraded.
It is therefore an object of the present invention to provide a method for enhancing voice quality of a CDMA communication system transmitting a voice signal and a signaling message by control of encoding rate of a variable rate vocoder.
In order to attain the above objects, according to an aspect of the present invention, there is provided a method for transmitting data by a variable rate vocoder included in a CDMA communication system. The data can include an encoded voice data and/or a signaling message. In accordance with the method, a mode of a present frame is detected, wherein the mode can be a normal mode for transmitting the voice data, a blank and burst mode for transmitting the signaling message, or a dim and burst mode for transmitting both the voice data and the signaling message. An encoding rate of a present frame is determined by reading out the voice data of the present frame, wherein the encoding rate can be a full rate, a half rate, a quarter rate, or an eighth rate. A counting value is increased when the determined encoding rate is the full rate. A packet to be transmitted in the present frame corresponding to the mode of the present frame is generated. A mode of a next frame is determined in response to existence of the signaling message to be transmitted in the next frame and the mode of the present frame, the packet is transmitted. These steps are repeated until all of the voice data and/or the signaling messages are transmitted.
In one embodiment, in detecting a mode of the present frame, it is determined whether the counting value is greater than a predetermined reference value when the mode of the present frame is the dim and burst mode. If so, a maximum encoding rate of the present frame is restricted to the half rate. If not, the maximum encoding rate of the present frame is restricted to the full rate. The counting value can represent a number of times of operation of the vocoder at the full rate. Alternatively, the counting value can represent an operating time of the vocoder at the full rate. The reference value can represent a permitted limit of an emergent state of the signaling message. Alternatively, the reference value can represent a response rate of the signaling message.
In one embodiment, the generated packet includes only voice data when the restricted maximum encoding rate is the full rate. The signaling message not transmitted with the voice data can be transmitted with other voice data if necessary when the encoding rate is changed from the full rate to another rate in the dim and burst mode. The generated packet can include both the voice data and the signaling message when the restricted maximum encoding rate is the half rate.
In one embodiment, in determining the mode of the next frame, it is determined whether the signaling message to be transmitted in the next frame exists. If not, the counting value is reset and the mode of the next frame is determined to be the normal mode. The maximum value is redefined when the signaling message exists. The mode of the next frame is determined to be the dim and burst mode when the present frame is the dim and burst mode. The mode of the next frame is determined to be the blank and burst mode when the present frame is the blank and burst mode.
According to the method of the invention, voice quality of the CDMA communication system can be improved, since the CDMA communication system transmits the voice data without restriction of the encoding rate of the vocoder and transmits the signaling message if necessary when a mode of the present frame is a dim and burst mode and an encoding rate is a full rate.